Composite materials of various kinds of fibers and matrix resins such as unsaturated polyester resins, phenolic resins, and epoxy resins are widely used in fields including building materials, sporting goods, leisure goods, and aircraft. Examples of fibers used in these composite materials include aramid fibers, glass fibers, carbon fibers, ceramic fibers, metal fibers, mineral fibers, rock fibers, and slug fibers. Among these fibers, carbon fibers are used for high-performance fiber-reinforced composite materials because carbon fibers have excellent tensile strength. Carbon fibers are usually produced in the form of filaments or tows, and processed, for example, into unidirectionally aligned sheets, filament winding, textiles, or chopped fibers for use. In the processing of carbon fibers, usually, a sizing agent is added to filaments or tows to prevent fluffing or breakage of yarn.
Composite materials having higher strength are in demand these days for use in various applications. Thus, developments have been carried out to improve the strength of composite materials by adding a new function to sizing agents.
For example, Patent Literature 1 discloses a composite material made of fiber bundles that have been sizing-treated with a sizing agent containing a specific epoxy compound. Patent Literature 2 discloses a composite material made of fiber bundles that have been sizing-treated with a sizing agent containing a rubber latex containing an epoxy group. Patent Literature 3 and Patent Literature 4 disclose composite materials made of fiber bundles that have been sizing-treated with a sizing agent containing blocked isocyanate.